Light emitting diodes (LEDs) are useful in various displays and especially in a new compact virtual display that utilizes a two-dimensional array of LEDs as an image source. Generally, these two dimensional arrays include large numbers of light emitting devices, from 5000 to 80,000 or more. A specific example exists where the image source consists of a high pixel count 2-dimensional array of LEDs, such as 240 columns by 144 rows, for a total of 34,560 pixels. An array the size of this specific example requires a total of 384 external interconnections to properly scan, or activate, and produce an image thereon. The array of LEDs is used to form complete images containing pictorial (graphic) and/or alphanumeric characters. The complete images are then magnified to produce virtual images which appear to an operator to be at least the size of a standard sheet of paper.
A major problem facing the productizing of such arrays is the penalty paid for this very large number of connection, or bond, pads required to provide information to the array. The foremost drawback is the increased semiconductor chip or chip area required for the connection pads and the interconnect fanout necessary to connect the connection pads to the rows and columns. A significant portion of the projected cost of the semiconductor chip on which the array is constructed is in the starting material and, with the 240.times.144 example set up for wire bonded external interconnects, the emitting region (light emitting diode array) occupies less than 20% of the total chip area with the remaining 80% required for connection pads and interconnect fan out. Conventional direct chip attach (DCA) bonding will improve this ratio only slightly because of the large pad sizes and interconnect pitches associated with the current state-of-the-art.
A large bonding substrate area is also required since a similar pad and interconnect fanout pattern must be repeated on accompanying semiconductor chips containing the drive electronics. Furthermore, the drive chips themselves must be large enough to accommodate the large number of connection pads (384 in this example). The net result is a large overall module which is not attractive for the applications of portable electronic devices where a premium is placed on small physical volumes.
One way to alleviate package size problems in LED display packaging is to simplify the package and assembly by integrating the LED display directly with the driver board, thereby minimizing the size requirement for both the display chip and the driver chip. Traditionally, there is provided a plurality of driver and controller circuits mounted on a substrate, or in the alternative, mounted on an optically transparent substrate, having data input terminals and further having control signal output terminals interfaced with the leads of the light emitting devices for activating the light emitting devices to generate images in accordance with data signals applied to the data input terminals.
In inorganic LED configurations, generally a semi-conductor substrate, or integrated circuit, is mounted on a printed circuit board or the like and the accepted method for connecting the substrate to external circuits is to use standard wire bond technology. However, when a semiconductor substrate having a relatively large array of electrical components or devices formed thereon is to be connected, standard wire bond techniques can become very difficult. For example, if a relatively large array (greater than, for example, 10,000 or 100.times.100) of light emitting diodes is formed on a substrate with a pitch (center-to-center separation) of P, then connection pads on the perimeter of the substrate will have a 2P pitch. This is true because every other row and every other column goes to an opposite edge of the perimeter to increase the distance between connection pads as much as possible.
At the present time wire bond interconnects from connection pads having a pitch of 4.8 mils is the best that is feasible. Thus, in the array mentioned above of 100.times.100 light emitting diodes the connection pads on the perimeter of the semiconductor chip would have a minimum pitch of 4.8 mils, with 50 connection pads situated along each edge of the perimeter. As more devices are included in the array, more connection pads are required and the perimeter size to accommodate the additional connection pads increases at an even greater rate. That is, since the minimum pitch of the bonding pads is 4.8 mils, the pitch of the devices in the array can be as large as 2.4 mils, or approximately 61 microns, without effecting the size of the substrate. Thus, even if the devices can be fabricated smaller than 61 microns, the minimum pitch of the bonding pads will not allow the perimeter of the substrate to be made any smaller. It can quickly be seen that the size of the substrate is severely limited by the limitations of the wire bonding technology.
Thus, there is a need for interconnect and packaging structures and techniques which can substantially reduce the limitations on size of LED display devices and semiconductor chips and which can reduce the amount of required surface area.
Accordingly, it is highly desirable to provide methods of fabricating LED arrays and interconnect apparatus packages which overcome these problems.
It is a purpose of the present invention to provide a new and improved method of fabricating LED arrays and interconnect apparatus packages.
It is a further purpose of the present invention to provide a new and improved LED array and integrated driver circuitry packaging for driving large arrays of LEDs.
It is another purpose of the present invention to provide new and improved integrated circuitry which requires less semiconductor chip area for larger arrays of devices.
It is another purpose of the present invention to provide a new and improved LED array and driver package with a substantially improved fill factor.
It is a still further purpose of the present invention to provide a new and improved method of fabricating LED arrays and driver packaging which is simpler and more efficient than prior methods and which is easily adaptable to high production levels.